Reactions of the hydrated electron with divalent aqueous transition-metal ions, Cd2+, Zn2+, Ni2+, Cu2+, Co2+, Fe2+) and Mn2+, were studied using a pulse radiolysis technique. The kinetics study was carried out at a constant pressure of 120 bar with temperatures up to 300 degrees C. The rate constants at room temperature agree with those reported in the literature. The reaction of Cd2+ is approximately diffusion-limited, but none of the first-row transition-metal ion reactions are diffusion-controlled at any temperature studied. The activation energies obtained from the Arrhenius plots are in the range 14.5-40.6 kJ/mol. Pre-exponential factors are quite large, between 1 x 10(13) and 7 x 10(15) M-1 s(-1). There appears to be a large degree of entropy-enthalpy compensation in the activation of Zn2+, Ni2+, Co2+, and Cu2+, as the larger pre-exponential factors strongly correlate with higher activation energy. Saturation of the ionic strength effect suggests that these reactions could be long-range nonadiabatic electron "jumps", but Marcus theory is incompatible with direct formation of ground state (M+)(aq) ions. A self-consistent explanation is that electron transfer occurs to excited states derived from the metal 4s orbitals. The ionic strength effect in the Mn2+ and Fe2+ reactions suggests that these proceed by short-range adiabatic electron attachment involving breakdown of the water coordination shell.